The present method relates to a novel method for affixing mineral fillers onto cellulose fibers in aqueous suspension. For example, affixation of calcium mineral fillers onto fibers may be implemented by chemical reaction entailing the precipitation of an insoluble chemical compound, such as calcium carbonate, which affixes itself to the cellulose fibers and fibrils. The cellulose fibers onto which these mineral fillers have been affixed are used in paper manufacture.
This invention applies to the entire paper industry. It applies, for instance, to the manufacture of papers ordinarily containing mineral fillers, for example printing and writing paper such as coated paper, printers"" paper such as newsprint, and lightweight coated paper (for magazines), and thin papers such as cigarette paper.
The invention also is applicable to manufacturing other kinds of paper such as sanitary and household papers that conventionally are free of mineral matters.
Cellulose fibers are short or long paper fibers. The aqueous fiber suspensions are prepared from an arbitrary pulp, namely chemical, bleached or unbleached, mechanical or thermomechanical pulps, or mixtures of these various pulps. Moreover, this pulp may also be derived by de-inking old papers or recycled papers.
Mineral fillers can be directly added to the manufacture of paper sheets.
Among the conventionally added mineral fillers, the most commonplace are the natural or synthetic calcium. carbonates (CaCO3). They are added to paper fibers to improve the features of the paper products. The mineral fillers may impart various properties to the paper. Due to their crystal structure and their particular morphologies, they impart whiteness, opacity, improved thickness etc. to the paper. They are economically significant because lowering the costs of the raw materials is cheaper than fibers.
However, they entail a capital difficulty consisting in affixing these fillers onto the cellulose fibers and, in particular, in the weakness of the filler-fiber bond. Typically, the fillers will not maintain contact with the fibers when paper sheets are being made. The fine mineral particles tend to move away from the fiber pad constituted by each sheet and a portion of these particles collects, whether recovered and/or rejected in the processing waters.
This phenomenon is commonplace when making absorbent papers based on cellular cotton, i.e. papers of low specific surface weight manufactured at high speeds, either conventionally, that is being dried or creped, or using a cross-blowing drying method.
Also, when a portion of the fillers is retained in the fiber pad, these fillers will spread non-uniformly through the thickness of the paper sheet.
Already retention agents have been added to improve mineral charge retention at the fibers for the purpose of overcoming the above problem.
Numerous publications and prior patents since 1945 describe procedures for precipitating mineral fibers onto fillers for the purpose of improving the fibers"" filler-retention and to avoid adding retaining agents. These procedures are based on such chemical reactions as addition or double decomposition. Some procedures in particular relate to precipitating mineral fillers into the hollow part of the fibers in order to preserve their mechanical properties and hence those of the paper, the properties generally being degraded by the fillers"" presence.
As a rule, the affixation method consists in introducing into an aqueous solution containing a substantial concentration of fibers, a first reagent based on one of the cations constituting the future precipitate, for example, calcium oxide or hydroxide or slaked lime.
After diluting the aqueous solution containing concentrated fibers and calcium hydroxide, and in the manner of the procedures disclosed in Japanese and French patent applications, respectively, JP 60 297,382 A (Hohuetsu Seishi) and FR 2,689,530 B1 (Aussedat Rey), carbon dioxide is injected to precipitate the calcium hydroxide.
On the other hand, International Application WO 92/15754 proposes injecting carbon dioxide in pressurized form into contact with the aqueous solution containing a high concentration of fibers for the purpose of affixing the precipitates simultaneously inside, on the hollow inner parts and in the walls of the fibers.
Other patents or patent applications disclose more complex procedures based on calcium salts. An additional stage eliminates one of the reaction products of the double decomposition. This is the case for U.S. Pat. Nos. 4,510,020 (Green) and 2,583,548 (Graig).
The latter patent describes a procedure wherein first the fibers are impregnated with calcium chloride in order to react this salt with sodium carbonate and then the mixture is washed to eliminate its sodium chloride.
U.S. Pat. No. 3,029,181 (Thomsen) discloses a similar procedure using ammonium carbonate.
International Application WO 91/04138 discloses a large number of mineral fillers which can be precipitated by a procedure using a double decomposition reaction.
However, all known precipitation procedures that have been described to date entail chemical and physical means which in turn require auxiliary preparatory stages, such as solubilizing the reagents, diluting or concentrating the aqueous fiber suspensions, and filtering or washing, in order to allow precipitation.
The stages much harper paper manufacturing procedures.
Illustratively, peripheral industrial"" equipment must be added to carry out the stages, namely mixing tubs, highly agitated, discontinuously operating reactors, filters, etc.
Such equipment is required on account of the frequently long chemical reaction times. As a result and in general, a production unit to prepare the filled fibers must be set up next to the conventional apparatus used in paper manufacture.
Consequently, the above described procedures of the prior art are infrequently used in industry and cannot compete with ex situ filler preparation.
Most of the present day manufacturing procedures still use previously prepared suspensions of minerals which are added to a suspension of fibers. In this case, the retaining agents are incorporated in order to retain the fillers on the fibers during paper manufacture.
The objective of the invention is to create a method allowing resolution of the problems of the prior art.
The objective of the invention is to eliminate adding retention agents and any accessory preparation stage for the purpose of integrating fillers xe2x80x9cin-linexe2x80x9d or xe2x80x9cin situxe2x80x9d into the overall paper manufacturing method.
For that purpose, the invention""s solution uses the waters of the paper manufacturing method as the reaction medium, that is, it uses the water contained in the aqueous fiber suspension, in the stage of affixing the mineral fillers onto the cellulose fibers.
Actually, the waters constitute a reservoir of ions and minerals which can precipitate.
Therefore, the invention involves using this reservoir of ions, which are in ionic equilibrium in the aqueous fiber suspension. The totality of the papermaking waters therefore constitute a single precipitation reaction medium.
In the description below, the waters are construed as xe2x80x9can aqueous solution of cellulose fibers resulting from paper makingxe2x80x9d.
This new method allows integrating in-line the precipitation stage of the mineral fillers onto the fibers as applied to papermaking taken in its widest sense. However, the method uses any of its waters without processing them in a special way. This method is widely applicable industrially and circumvents using additional retention agents.
In an essential feature of the invention, the method affixing a mineral filler onto cellulose fibers in an aqueous solution includes using an aqueous suspension of cellulose fibers derived from papermaking and comprising at least hydrogen carbonates, or carbonates or silicates of alkali metals and/or earth alkali metals, as the reaction medium to which are added a hydroxide of the mineral filler in order to precipitate carbonates or silicates of the mineral filler onto the fibers.
In a preferred feature of the invention, the aqueous cellulose fiber suspension comprises sodium hydrogen carbonates.
In another feature of the invention, the aqueous cellulose fiber suspension furthermore contains calcium and/or magnesium hydrogen carbonates.
In another feature of the invention, the full alkalimetric titer of the aqueous suspension is between 2and 30xc2x0 F.
In another feature of the invention, the mineral filler hydroxide is a calcium hydroxide.
Another object of the invention is a papersheet manufacturing process.
In an essential feature of the invention, this process involves:
(a) preparing or providing a manufacturing composition based on water and a bleached or unbleached paper fiber pulp, a mechanical or a thermomechanical pulp or their mixtures, comprising, in ionic equilibrium, at least ions of alkali metals and/or earth alkali metals and ions of hydrogen carbonates, carbonates or silicates;
(b) adding to the manufacturing composition a mineral filler hydroxide to affix the mineral filler onto the paper fibers, and
(c) forming a wet paper sheet on a papermaking machine based on the paper fibers which were loaded in this manner in suspension, and drying the sheet.
In an additional feature of the invention, the process furthermore includes:
(d) recovering the drip waters of stage (c) and injecting into them a gas containing carbon dioxide to stabilize their pH, and
(e) recycling the waters processed in that manner into the manufacturing composition of stage (a).
In one advantageous feature of the invention, the manufacturing composition is based on pulp derived from old de-inked papers.